Download Part 3: Normal faults and extensional tectonics

12.113 Structural Geology
Part 3: Normal faults and extensional
tectonics
Fall 2005
Contents
1 Reading assignment
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2 Growth strata
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3 Models of extensional faults
3.1 Listric faults . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Planar, rotating fault arrays . . . . . . . . . . . . . . . .
3.3 Stratigraphic signature of normal faults and extension
3.4 Core complexes . . . . . . . . . . . . . . . . . . . . . . .
4 Slides
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1 Reading assignment
Read Chapter 5.
2 Growth strata
Although not particular to normal faults, relative uplift and subsidence on either
side of a surface breaking fault leads to predictable patterns of erosion and sedi­
mentation. Sediments will fill the available space created by slip on a fault. Not only
do the characteristic patterns of stratal thickening or thinning tell you about the
1
Figure 1: Model for a simple, planar fault
style of faulting, but by dating the sediments, you can tell the age of the fault (since
sediments were deposited during faulting) as well as the slip rates on the fault.
3 Models of extensional faults
The simplest model of a normal fault is a planar fault that does not change its dip
with depth. Such a fault does not accommodate much extension. (Figure 1)
3.1 Listric faults
A listric fault is a fault which shallows with depth. Compared to a simple planar
model, such a fault accommodates a considerably greater amount of extension for
the same amount of slip. Characteristics of listric faults are that, in order to maintain
geometric compatibility, beds in the hanging wall have to rotate and dip towards the
fault. Commonly, listric faults involve a number of en echelon faults that sole into a
low­angle master detachment. (Figure 2)
3.2 Planar, rotating fault arrays
An array of parallel, normal faults slipping on a sub­horizontal detachment will ro­
tate with fault slip, much like books on a book shelf. As they rotate, the dip of the
faults will get shallower, and the beds between the faults will also rotate. Again, the
amount of extension can be calculated for a given angle between beds and the fault,
and the total amount of slip on the fault.
Sometimes, normal fault systems are, in fact, best described as a combination
of listric faults with rotating, planar faults bounding rotating blocks in the hanging
wall. The "master fault" – called a detachment fault is a listric fault, with a set of
rotating planar faults in its hanging­wall or upper plate. The steep part of the de­
tachment is sometimes called the break­away fault. Figure 4
3.3 Stratigraphic signature of normal faults and extension
If, prior to extensional faulting, stratigraphy was undeformed, then the characteris­
tic of normal faulting is that younger rocks will be juxtaposed over older rocks, but
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Figure 2: Geometry of listric normal faults. The bottom figure shows the geometric
relations for calculating the amount of extension, given the angle between beds and
the fault, and the total fault slip
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Figure 3: A system of rotating, planar faults. The bottom figure shows how this ge­
ometry compares to simple listric normal faults in terms of the amount of extension
that can be produced
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Figure 4: Listric fault, with rotating upper plate blocks and breakaway fault
Figure 5: Stratigraphic signature of extensional tectonics where low angle faults are
present
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Figure 6: Model for core complex formation along a low angle detachment
with "missing section" at the fault.
Particularly where the normal faults are low­angle, this relationship is often mis­
interpreted. Prior to the recognition that low angle normal faults are widespread
features of the extended crust, they were often mapped as thrusts (which makes no
sense) or unconformities. Where low angle faults are common, a "stratigraphic sec­
tion" will show many apparent gaps. Figure 5
3.4 Core complexes
A common feature of highly extended terrains is that large amounts of extension
on relatively low angle normal faults juxtaposes rocks of high metamorphic grade
against unmetamorphosed rocks and even surficial deposits. The exposure of high
grade rocks typically domal, or antiformal geometry, with normal faults completely
bounding the high­grade rocks. Rocks in the immediate footwall of the bounding
normal faults show a characteristic series of fabrics: since rocks are brought up from
depth, fabrics reflecting progressively colder and more brittle deformational envi­
ronments are superimposed upon one another. So, walking from the interior of the
core complex out towards the low­grade upper plate rocks ductile fabrics and my­
lonites will be overprinted by brittle­ductile transitional fabrics, which themselves
are often overprinted by thick gouge and breccia and then a discrete, brittle fault.
The conceptual model of core complexes – high grade rocks brought to the sur­
face along low angle detachments has some important implications for how exten­
sion is accommodated in the lower crust or mantle. In figure 6, large amounts of slip
on a detachment obviously creates a great deal of "space" in the lower crust. The
question then is: does the detachment offset the Moho, and accommodation is then
6
Figure 7: Two models for accommodating large amounts of extension in the upper
crust. From Block and Royden 1984
taken up by flow of mantle rocks. Or does the detachment sole into the middle or
lower crust, with lower crustal flow solving the space problem. See figure 7.
4 Slides
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Figure 8: Rocks above a low angle detachment are broken up by normal faults that
sole into the main detachment. The yellow unit is the same in all the outcrops
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Figure 9: Range scale tilted normal fault block
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Figure 10: Series of planar rotated normal faults. Scale is about 2 meters from top to
bottom of the slide. This faulting has accommodated about 60% extension.
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Figure 11: Listric fault
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Figure 12: Listric fault merging into a detachment surface at the feet of geologist.
Figure 13: Low angle normal fault. Some people still don’t believe these are real.
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Figure 14: Low angle normal fault; rocks in the hanging wall of the fault are late
Quaternary. There are folks who believe that low angle faults do exist, but believe
they were rotated from an initially steep attitude. This a more reasonable attitude
than total denial, to be sure.
Figure 15: The Whipple mountains are one of the archetypical core complexes of the
western U.S. extensional provinces. This is a cross­section from the work of Lister
and Davis. Note the multiple rotations.
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Figure 16: A view looking east at the northern Panamint Rnage: lower plate high
grade rocks to the left, upper plate rocks to the right. The surface sloping to the left
in the slide is the detachment fault surface.
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Figure 17: View to the east along the Whipple detachment. Dark rocks are Tertiary
aged sediments and volcanic rocks in tilted and rotated blocks. Pale rocks are the
lower plate mylonites
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Figure 18: Spot the low angle detachment
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Figure 19: Detachment surface in the Clark mountains, California
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Figure 20: High grade fabrics representative of extensional strain at deep crustal lev­
els. These rocks are lower plate rocks below a detachment in Death Valley.
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Figure 21: High grade fabrics with lower grade (i.e. progressively more brittle) defor­
mation overprinted on them.
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Figure 22: High grade extensional shear fabrics
Figure 23: C/S fabric, ductile deformation
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Figure 24: Low grade (brittle) gouge and breccia; lower plate of a detachment fault.
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